It is well known that the failure rate for an electronic device such as a semiconductor memory is largest during a short period just after the commencement of employment thereof and decreases exponentially following the period in which it is employed thereafter. A burn-in test or an acceleration test is a test to selectively exclude electronic devices having initial faults by applying an electronic device such as a semiconductor memory with an electric voltage higher than the rated voltage of the electric device to impose the electric device to be tested with conditions severer than the ordinary conditions at which the electric devices are ordinarily employed. In other words, the burn-in test or the acceleration test is a test to cause an electronic device to endure intentionally produced virtual employment conditions equivalent to those which cause results as if the tested electronic devices are employed for a long period.
For example, an electronic device of which the rated power supply voltage is 5 V and which is employable under an ambient temperature of zero through 70.degree. C. is applied, under a burn-in test, 7 V at an ambient temperature of 125.degree. C. When successfully passing such a burn-in test, the electronic device is determined to be a good product employable for a guaranteed period under rated conditions.
In a burn-in test applicable to a semiconductor memory available in the prior art, each word line is selectively applied a voltage higher than the rated power supply voltage in turn and during the period, all of the bit lines are applied the same higher voltage off and on simultaneously with the application of a higher voltage to the word lines. This means that the period required for a burn-in test is equivalent to a period calculated by a formula (the period of application of voltage to all the bit lines and the period of pause) multiplied by (the quantity of the word lines). Supporting a four Mega-bit RAM being 2048 word lines and 2048 bit lines, and supposing the period of application of voltage to all the bit lines and the period of pause is one minute, an approximate period of 34 hours is required to finish a burn-in test for one four Mega-bit RAM. Since this period is long, it is required to shorten the period required for conducting a burn-in test applicable to a semiconductor memory. In this sense, development of a semiconductor memory having a built-in circuit employable for conducting a burn-in test or an acceleration test is a remarkable advantage for the industry.